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Unread 02-29-2016, 06:29 PM   #1
wiz02
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5mm Underlayment

Hi:

I'd like to get your opinions on adding a 5mm (nominal 1/4") plywood underlayment to the existing 3/4" plywood subfloor which is in good shape other than having lots of staple holes after removing the original vinyl flooring and 1/4" underlayment.

If I add the 5mm underlayment to the 3/4" plywood subfloor, then the total thickness of the tile + 1/4" CBU is within 1/32" of the hardwood that the laundry room transitions to. (I don't know if you allow additional thickness for mud or not but I assume it's not much).

Note that I'm very conscious of mismatched floor height transitions as my spouse still comments about the crappy job done by the contractors at a relative's house, where they ended up with about a 2" transition from the kitchen to the living room / dining room. (which was done close to 10 years ago).

FYI: The floor joists in my laundry room are 2" x 10" and the span is only 7', 16" on center so there is no deflection issue given L / 1813 according to the deflect calculator.

I am also planning to use porcelain tiles that look like wood planks.

Appreciate any and all advice. Thanks!
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Unread 02-29-2016, 06:54 PM   #2
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Wiz, I suspect your span is greater than 7' unless you have walls directly below both laundry room walls. Span is determined by direct support of floor joists. Best viewed and measured from below...crawl space?

As to gaining 1/4", The 5mm has no place in a tile assembly, minimum being 1/2" ply. Why not 1/2" tile backer?. BTW, a 2" height difference is huge, no wonder your wife noticed it. On the other hand a 1/4" difference is negligible, especially if it's carpet adjoining.
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Unread 02-29-2016, 07:24 PM   #3
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Hi Peter:

Thanks for the quick response. The house has a steel support beam 7 feet from the foundation wall, so the floor joist span is definitely 7', as the laundry room is directly above this area.

My concern with going with 1/2" CBU is that if down the road we want to change the tiles, I'm going to have to replace the 3/4" subfloor, as I doubt I will be able to clean off the thinset if it's applied directly to the 3/4" subfloor. Therefore, I was thinking that if I add 5mm underlayment with 1/4" CBU, I can always remove the two and the original 3/4" subfloor will be relatively intact.

Would you mind explaining your statement that "The 5mm has no place in a tile assembly" given that there is is solid subfloor beneath it? I've seen people say the same thing is in other forums and I don't get why this would be the case if the original subfloor met deflection requirements. What's the difference between the thinset bonding to the first layer of 3/4" plywood or the first layer of 5mm?

Thanks again for your response, just trying to plan ahead and learn without having to redo the job.

HW
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Unread 02-29-2016, 07:44 PM   #4
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Hmmmm, if you use that 5mm, there's a better than good chance the floor will remove itself in time. So in that regard, your future demo would be easier! Sorry I couldn't resist

In a word...err, two words, no rigidity. Unless you can make a perfectly mated 100% glued interface between subfloor and that underlayment, what you're proposing is a recipe for failure. It's harder to do than you might guess. Anyway, if you went the glued route it'd mess up your future plan for demo.

If you want something easy to change, vinyl is a better bet.
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Unread 02-29-2016, 07:49 PM   #5
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OK - then I'm throwing out the future demo concept and going with 1/2" CBU.

Thanks ever so much for the explanation and for saving me from myself!
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Unread 02-29-2016, 07:55 PM   #6
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Plywood that thin turns into a wavy potato chip when exposed to moisture. Even with an abundance of fasteners, and fastened below cement board, it still gets wavy. The thicker the ply, the less this happens. Even 3/8" plywood (the minimum thickness of plywood used in a tile assembly) is somewhat problematic. So stay away from the thin stuff. You mentioned 5mm...are you referring to Baltic Birch?

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Unread 02-29-2016, 09:53 PM   #7
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Haven't heard the term Baltic Birch in quite a while, however, this is some Chinese version from HD, with no mention of birch, but it does look nice. However, it's going back in exchange for 1/2" CBU. However, if I can convince my wife that a 1/4" floor height difference between the ceramic tile and hardwood flooring is OK, I'll go for 1/4" CBU, but bottom line, I'm going with whatever makes the customer happy.
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Unread 02-29-2016, 10:11 PM   #8
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Welcome, Wiz.

If you have a thickness concern, seems like the last thing you want on your floor is 1/2" CBU. The CBU manufacturer will recommend his thinner product for any floor application.

If you want to add anything for structure, you want to add a second layer of nominal half-inch plywood and then the thinner CBU or an even thinner membrane-type tiling substrate.

Suitable transitions from hardwood to ceramic tile are among the easiest to make in my experience.

My opinion; worth price charged.
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Unread 03-01-2016, 05:28 PM   #9
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Thanks for the welcome. I don't have a thickness concern regarding the subfloor, but I do want to match the finished height of the tile as closely as possible to the standard strip oak flooring thickness at the transition which is 3/4 of an inch.

What's your recommendation for a transition to the ceramic tile (actually porcelain wood plank look a likes). Your suggestion is to go with 1/4" CBU and the tile is nominally 1/4" thick which leaves a 1/4" height difference which I can probably sell to my spouse, but how do you recommend I handle the transition?

I had a oak transition strip that had a groove for the tongues in the oak strips than curved down from the full 3/4" thickness to a thin edge. The whole transition inserted into the end of the wood flooring and rested on top of the vinyl. Going with 1/4" CBU, the tile will be 1/4" below the top of the oak, roughly even with the top of the tongue. What should I use to cover the tongues of the wood flooring and the tile that butts up to it? Thanks for the advice (no matter what the price!) If I should have searched, just let me know as I started writing and probably should have searched first.
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Unread 03-01-2016, 06:56 PM   #10
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Wiz, if your half-inch CBU gives you the height you want, that's fine. My recommendation was meant to give you a better subfloor than your current damaged single layer and would put your tile installation higher than your hardwood.

But transitioning from the hardwood to the tile will work in either direction. I favor making a transition strip to closely match the existing and be flush with the hardwood on one side and flush with the tile on the other.

Entirely up to you how you want to handle your situation, though.

My opinion; worth price charged.
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Unread 03-01-2016, 07:26 PM   #11
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If you have a total thickness of 3/4", you might not even want any additional thickness, depending on your tile thickness. If you figure on 1/8" of thinset under the backer board, and another 1/8" between the backer board and the tile, you are already up to 1/2", which only leaves 1/4" for your tile.
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Unread 03-01-2016, 09:43 PM   #12
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I was wondering what the rule of thumb is for thinset, now the 1/4" cbu is sounding even better, given that the current 3/4" subfloor is really in good shape. I'm enough of a woodworker to rig up a transition, especially after I used that wonderful, sometimes under used function known as search
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Unread 03-01-2016, 10:42 PM   #13
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Exactly how much difference is there between the height of the 3/4" subfloor and the wood flooring in the adjacent room?

We might be able to give you some suggestions about certain materials that would bring your new floor to the height you want. Knowing the thickness of the tile to be used would be helpful.
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Unread 03-02-2016, 12:27 AM   #14
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Depending on the exact thicknesses, DitraXL works to make many tile+underlayment combinations equal to typical 3/4" wood substrates.
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